Computers and other electronic apparatus containing heat-generating microprocessors or computer chips, as well as other heat-generating components, require cooling of those components when the system is operating. Preferred compactness of electronic systems, such as computers, sophisticated recognition systems, advanced sensing systems, aiming/targeting systems, display systems, mapping systems and the like leads to a high density of heat generating electronic components. Moreover, the high speed and efficient components used in the latest systems generate significantly more heat than used in systems of just a few years ago. For example, microprocessors as used in present office and home computers can generate over 80 watts of heat energy, more than twice the amount of heat generated by previous home computer processors. To facilitate the use of multiple high level heat generating electronic components, chip cooling and electronic board cooling techniques have been developed. Most commonly, outside air is induced into the housing with one or more fans and the airflow is used to cause convective heat transfer thereby cooling the components before the air returns to the outside of the housing. Fans require additional energy to operate, are rather bulky taking up substantial amounts of interior space, and the resulting airflow is often difficult to direct to heat specific electronic components constituting the actual heat sources, especially when cooling needs change due to variations in computing load, display load, sensing load or ambient conditions.
The reliable functioning of computer and other electronic components requires operation within certain temperatures boundaries. For example microprocessor chip temperatures frequently must be operated below 60° C. for reliability and long life. Display systems require minimum temperatures for the display to be easily visible and cannot tolerate extreme heat without losing reliability and ultimately functionality. Most of these components generate heat while they are operating, and the amount of heat generated depends on the activity level and can vary significantly. Some components can produce more than two or three times their common or low end activity heat output; some components may not operate for long time periods and require essential no cooling until they are operational. Other components, especially liquid crystal displays also require a minimum temperature for proper operation. If such systems are to be used for example in cold winter climates with ambient temperatures below freezing or even below −40° C., the start-up of the display may require heat input to facilitate a minimum operating temperature as specified by such component.
Often computers and other electronic systems are not operated continuously, 24/7, or at least not with the same intensity at the same ambient temperature conditions, but instead are used either intermittently or selectively, often for relatively short periods of time with substantial inactive periods between uses or more intensely during some periods than others. Thus, cooling of the heat-generating components may vary in required cooling capacity over time or only be required for short-term, high-power heat rejection. These conditions pose a problem in applications in which the total heat rejection and cooling capacity is limited or at least limited for certain time periods.
Presently designed heat rejection systems are often inadequate to meet the load requirements needed to satisfy increasing demands for more versatile and faster operation and more complex computational schemes and display systems. At certain times the thermal capacity of present heat rejection system is overloaded to a degree and for a time period sufficient to result in electronic component failure. Moreover, all components may not fail, but rather those components that were furthest away from their allowable operating temperature condition or one or more of those components that can not tolerate excessive operating temperatures during certain or all of its operating function. Operation in extreme ambient conditions as often encountered in military applications puts additional burden on the heat transfer as the available differential temperature to the ambient is reduced.